1. Field of the Invention
The present invention relates to a noise eliminating circuit for eliminating a noise component from a color TV signal (a chroma signal, or a color difference signal such as an R-Y or B-Y signal, or the like).
2. Description of the Prior Art
FIG. 1 is a block diagram of a conventional noise eliminating circuit of the type described above. FIG. 2 shows waveforms for explaining the mode of operation of the circuit shown in FIG. 1. Referring to FIG. 1, a 3.58 MHz input chroma signal (FIG. 2A) is delayed as shown in FIG. 2B by a 1-H delay line 1 (H:horizontal scanning period) constituting, together with a subtractor 2, a comb-filter. Since the color subcarrier of the NTSC system have the opposite phases for each successive line, the chroma components of the input to and output from the 1-H delay line 1 have the opposite phases and are line correlated. The output from the 1-H delay line 1 is subtracted from the input chroma signal by the subtractor 2 to reduce its output level to 1/2 the original level. Then, a chroma signal having a line correlation as shown in FIG. 2C is obtained, and a noise or distortion component (N) which is not line correlated is cancelled out.
The level of the front end portion of the chroma signal as an output signal from the comb-filter comprising the 1-H delay line 1 and the subtractor 2 thus becomes A/2 as shown in FIG. 2C. Furthermore, a chroma component having a level of A/2, which component is not included in the original input signal, appears at the back end portion of the chroma signal. For this reason, color smear in the vertical direction of the screen is caused, and the color level changes at the edge of the image.
In order to reduce this undesirable effect, the following signal processing is performed. The input chroma signal is subtracted by a subtractor 3 from an output (FIG. 2C) from the comb-filter so as to extract a signal component having no line correlation (front and back end portions of the chroma signal) as shown in FIG. 2D, and to extract a noise or distortion component having no line correlation. The extracted signal is supplied to a noise cancellation circuit comprising a limiter 4 and a subtractor 5. In the noise cancellation circuit, the limiter 4 limits the amplitude of the signal component alone so as to amplitude-separate the signal components and the noise or distortion component as shown in FIG. 2E. The subtractor 5 subtracts an output signal E from the limiter 4 from an output signal D from the subtractor 3. Thus, as shown in FIG. 2F, a noncorrelated chroma signal having no noise or distortion component is extracted.
The noncorrelated chroma signal which is extracted by the noise cancellation circuit is added to an output (FIG. 2C) from the comb-filter by an adder 6, thereby performing signal level correction at the front and back end portions of the chroma signal as shown in FIG. 2G. The S/N ratio of the corrected chroma signal is improved by about 3 dB with respect to the input chroma signal, and the signal distortion (change in level) at a front end portion F and a back end portion B is reduced in comparison with a case wherein a comb-filter alone is used.
However, as shown in FIG. 2G, the problems of reduction in the color level at the front end portion F of the signal and color smear at the back end portion B remain. In order to eliminate these problems, the limiting level of the limiter 4 of the noise cancellation circuit shown in FIG. 1 must be increased (that is, the limiting function must be enhanced with respect to the chroma signal). Conversely, in order to reduce the residual noise level and distortion component, the limiting level must be decreased (that is, the limiting function must be weakened).